C172 INSTRUCTOR AND STUDENT DECLINED TO CLB TO HIGHER ALT BECAUSE OF POSSIBLE ICING AT A COLDER TEMP IN ZBW CLASS E.

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I WAS GIVING INST TRAINING TO A STUDENT. WE HAD BEEN IN AND OUT OF IMC/VMC AT ALTS OF 6000 FT AND BELOW FOR APPROX 3 HRS PRIOR; DURING THE MORNING'S PREVIOUS FLTS. TEMPS STAYED POSITIVE THE ENTIRE TIME WITH NO ICING OBSERVED. SAME CONDITIONS EXISTED ENRTE TO THE IAF AT 6000 FT. SINCE WE WERE GPS EQUIPPED; AND THE MINIMUM IFR ALT FOR THE REMAINDER OF THE FLT WAS 6000 FT; I EXPECTED NO ICING PROBS. CTR CTLR CLRED THE ACFT UP TO 6800 FT AND STATED THAT THIS WAS HIS MINIMUM VECTORING ALT. TEMP AT 6000 FT WAS NOW +1 DEG C. I ADVISED CTLR THAT IT WOULD BE A SLOW CLB AND THAT I WOULD RATHER STAY AT 6000 FT AND AVOID ANY POSSIBLE ICING CONDITIONS. ALTHOUGH NOT SPECIFICALLY CLRED TO REMAIN AT 6000 FT THE CTR CTLR STATED THAT I WAS ABOVE THE MINIMUM IFR ALT AND WELL CLR OF OBSTACLES. WE WERE LATER CLRED TO 5000 FT AND CLRED APCH. WE BROKE OUT INTO VMC AT 5000 FT AND PROCEEDED TO THE ARPT VISUALLY. THE PHRASEOLOGY GIVEN BY CTR LEFT SOME DOUBT IN MY MIND WHETHER AN ALTDEV ACTUALLY OCCURRED. IN RETROSPECT; IT SEEMED PRUDENT TO DECLINE THE CLB INTO POSSIBLE ICING CONDITIONS. TERRAIN AND TFC CLRNC WAS NOT AN ISSUE.